This invention relates to ink compositions for ink-jet printers, and, more particularly, to a process for preparing acidic aqueous-based inks.
The use of aqueous-based inks for ink-jet printers is well-known. Such compositions are relatively inexpensive and easy to prepare; typically, the ink comprises water and a aglycol ether, usually diethylene glycol, and a dye. Commonly, the water and glycol ether are present in generally the same proportion and the dye, for example, Food Black 2, is present up to about 6% of the total composition, depending on the desired density of the print.
However, a persistent problem associated with aqueous-based inks is their propensity to crust over a period of time, eventually leading to plugging of the orifice in the printer mechanism from which droplets of ink are expelled in the printing operation. The crusting problem arises from the evaporation of the water from the ink solvent (vehicle) and the consequent precipitation of the dye salt which has become substantially insoluble as a result of this water loss.
The prior art inks generally use existing commercial dyes salts (cation plus dye anion) as formed, which are simply dissolved in the vehicle and filtered to prepare the ink. Such dyes are designed to form solids in paper or cloth, employing such cations as sodium cations, which promote precipitation of the dye salt. Consequently, the dyes do not easily remain liquid in the orifice of an ink-jet printer.
Attempts have been made to solve the crusting problem. Hygroscopic agents have been added to reduce the rate of water evaporation by their ability to pick up water vapor from the air. Exemplary of such hygroscopic agents are water-soluble polymers, alkanol amines, amides and polyhydric alcohols.
While some improvement has been realized with these hygroscopic agents, a total solution to the crusting problems has not yet been achieved. Further, apparently no methods are known to prevent precipitation of the anionic dyes (acid dyes) in acidic aqueous-based inks.